Ischemic tolerance is a phenomenon whereby a sub-lethal ischemic injury, preconditioning, induces endogenous protective mechanisms that lessen the impact of a subsequent, more severe ischemic insult. We have used modeled ischemic tolerance in rat and mouse, both in vivo and in vitro, to describe multiple effectors of neuroprotection in these endogenous systems. Recently, we used DNA microarray to examine changes in gene expression in mouse brains subjected to preconditioning induced by a brief duration of ischemia, injurious ischemia, or a tolerant brain (preconditioned, then challenged with injurious ischemia). These experiments revealed that injurious ischemia up-regulated gene expression, while ischemic tolerance resulted in significant down-regulation of gene expression. Thus the signature of ischemic tolerance is that of suppressed gene expression. Elucidating the molecular mechanism(s) that underlie this focused transcriptional suppression is our goal. Eukaryotic gene expression is regulated by microRNAs, a newly identified class of small non-protein coding RNAs that regulate mRNA translation and chromatin activity in mammalian cells. Based on the signature of ischemic tolerance, a suppression of gene expression, we hypothesize that ischemic tolerance leads to regulated microRNA expression that in turn leads to the protected phenotype. In support of this hypothesis, our preliminary data show that distinct subsets of microRNAs are regulated in preconditioned, ischemic, and tolerant mouse brain. Thus to fully examine a role for microRNAs in ischemic tolerance, we propose the following specific aims: 1) establish the expression profile of microRNAs in preconditioned, ischemic, and tolerant mouse brain by microarray analysis, 2) confirm changes in, and examine the temporal expression of, microRNAs regulated in preconditioned, ischemic, and tolerant mouse brain, and 3) examine the effect of regulated microRNAs on target protein expression and cell survival. These studies are among the first to examine the regulated expression of miRNAs in response to modeled ischemia, and given the current development of short, interference RNAs as novel therapeutic agents for a number of diseases including macular degeneration, asthma, diabetes, cancer, Huntington's, and Hepatitis C infection, these studies may provide rationale for the development of similar strategies for the treatment or prevention of stroke and brain ischemia. [unreadable] [unreadable] [unreadable]